Armature doesn't properly bend arm mesh
At first glance, there are multiple reasons why your rig could be bending sharply, but if you try moving the upper arm, things should become a little clearer.



Your model has weird bends because of your automatic bone envelopes rig.

The vertices in the range of the bone envelope need to slightly affect the vertices outside of the envelope as well, producing a falloff.

You are using a modifier with automatic bone envelopes, so you will have to manually weight-paint the bone influences, or assign vertices manually with vertex group rigging. 

It is going to be tedious due to the sheer number of vertices your model has. However, you can make rigging easier for yourself in the future by following a very important rule: BE CAREFUL WHEN APPLYING SUBDIVISION SURFACE MODIFIERS.

Applying a subdivision surface modifier is destructive and cannot be reverted. You will have to make your 3D model from scratch if you are unable to weight paint every single vertex to each bone or use the vertex group method.

The cubes in the top image are all the same model; the desired result has an un-applied subdivision surface modifier, and the sharp bend result has an applied subdivision surface modifier. It is better to work with un-applied subdivision surface modifiers since there are less vertices to select and navigate around when rigging manually, which you will have to do to fix your rig. 

Automatic rigging options, such as the checkbox in the armature modifier, or parenting a mesh to an armature with automatic weights, sadly, do not always work.